This invention relates to a process for making a coil, and more particularly, to a process for making a coil having no bobbin, such as a large solenoid coil for a detector for use in the high energy physical experimentation or a coil for a nuclear fusion reactor.
FIG. 1 illustrates an example of a conventional process for making a toroidal field coil which is disclosed in an article entitled "The D-shaped Main Field Coil for the ASDEX Fusion Experiment at the Max Plank Institute for Plasma Physics at Garching near Munich" in Brown Boveri Review, Volume 64, of Feb. 2, 1977.
The conventional process for making a coil comprises the steps of connecting one end 6 of an electrical conductor 1 between two of the base segments 3a which together constitute a D-shaped jig 3, rotating the jig 3 in the direction of arrows 4 to wind the conductor 1 thereon while applying a constant tension T to the conductor 1 by means of a tension generating unit 2, so that the conductor 1 is wound on the base segments 3a of the jig 3, removing the thus formed coil from the jig 3 and applying an insulating varnish on the outer surface of the coil.
Generally, in order to cohere coil windings to each other, a varnish treated insulation is inserted between the coil windings or is wound around the conductor 1, heated to cure the insulation and the coil windings and the coil is finally finished.
According to the conventional process as above described, the coil winding is carried out while applying a constant tension to the conductor 1 by means of the tension generating unit 2. When the coil is removed from the coil making jig 3, the windings of the coil are slightly diametrically decreased as a whole, so that the residual stress by the tension remaining in the conductor 1 becomes almost null.
In general, since the tension generating unit 2 is not very precise, a tolerance of about .+-.20% in tension results in the windings of the conductor 1. Accordingly, a residual stress corresponding to this tolerance exists in the conductor 1, and this residual stress produces a force to slide the turns constituting the coil windings which are adjacent to each other in different directions. When the turns of the conductor 1 slide with respect to each other, the rigidity of the coil becomes so weak that the coil deforms easily due to stresses such as electromagnetic forces generated during the operation of the coil. In the worst case, the coil can be mechanically and electrically destroyed.